In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Moreover, a cell area for a base station may be divided into a plurality of sectors (also referred to as cells) surrounding the base station. For example, a base station may service three 120 degree sectors/cells surrounding the base station, and the base station may provide a respective directional transceiver and sector antenna array for each sector. Stated in other words, a base station may include three directional sector antenna arrays servicing respective 120 degree base station sectors surrounding the base station.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999). Performance may be improved if both the transmitter and the receiver for a base station sector are equipped with multiple antennas (e.g., an sector antenna array) to provide a multiple-input multiple-output (MIMO) communication channel(s) for the base station sector. Such systems and/or related techniques are commonly referred to as MIMO (Multiple-Input-Multiple-Output). The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink (DL) from a base station transmitting from a sector antenna array over a MIMO channel to a wireless terminal in the sector, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from the base station sector antenna array for the sector. Stated in other words, multiple symbol streams may be transmitted from the base station sector antenna array for the sector to the wireless terminal over the same downlink time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station sector transmitting from the same sector antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station sector antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station sector antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
To further increase throughput at a sector/cell edge (also referred to as a soft handover area or border area) using High Speed Downlink Packet Access (HSDPA), Multi-Flow-HSDPA (MF-HSDPA, also referred to as Multi-Flow-HSDPA or MP-HSDPA) has been proposed for 3rd Generation Partnership Project (3GPP) communications. In MF-HSDPA, transport data blocks of a data stream may be transmitted from two different sectors/cells of the same or different base stations to a same wireless terminal in a border area between the sectors/cells. Intra NodeB aggregation (also referred to as intra node Multi-Flow communications) occurs when different transport data blocks of a data stream are transmitted from two different sectors of a same base station to a wireless terminal, and Inter NodeB aggregation (also referred to as inter node Multi-Flow communications) occurs when different transport data blocks of a data stream are transmitted from sectors/cells of different base stations to a wireless terminal. MF-HSDPA may thus provide advantages of parallel data streams like MIMO where the spatially separated antennas are taken from different sectors/cells.
In the opposite direction, uplink transmissions from the UE may be transmitted to two different sectors/cells of the same or different base stations when located in a soft handover area (or border area) between the sectors/cells. At 3GPP (3rd Generation Partnership Project) RAN (Radio Access Network) #54 plenary meeting, a work item (WI) on MIMO with 64-QAM (Quadrature Amplitude Modulation) for HSUPA was initiated. See, 3GPP TSG RAN Meeting #54 RP-111642, “MIMO With 64QAM for HSUPA”, Berlin, Germany, Dec. 6-9, 2011, the disclosure of which is hereby incorporated herein in its entirety by reference. The WI initialization was a result of studies regarding potential benefits and solutions performed during the study item (SI) phase. For a summary of the findings, see, 3GPP TR 25.871, V11.0.0, “Uplink Multiple Input Multiple Output (MIMO) for High Speed Packet Access (HSPA),” Release 11, 2011-09, the disclosure of which is hereby incorporated herein in its entirety by reference.
For uplink MIMO transmissions to multiple sectors/cells during soft/softer handover, the radio access network may use a serving grant (essentially a power measure) to control the interference that a UE is allowed to create. The serving grant thus gives an “upper bound” on how much data the UE may transmit. For dual stream (rank 2) uplink MIMO transmissions, a transport format combination for the primary stream may be essentially controlled by the serving grant, while a transport format combination for the secondary stream may be essentially controlled by a combination of the serving grant and an offset value that may be determined by the serving base station (nodeB). If the multiple sectors/cells for the SHO communications are at different base stations (nodeBs), however, the serving grant and/or offset value may not adequately reflect different channel characteristics between the UE and the different sectors/cells.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.